1. Field of the Invention
This invention relates generally to electron emitting materials and more particularly, it relates to single crystal lanthanum hexaboride electron emitters.
2. Description of the Prior Art
Many modern instruments, such as scanning electron microscopes and electron beam exposure systems, require small but bright and dimensionally stable electron sources. The cathode material used as a source of electron beams has usually been either tungsten or thoriated tungsten. Both materials are relatively bright and sources fabricated from these materials have reasonable lifetimes and have performed well. However, as the capacity demanded of these instruments has increased, other cathode materials with longer lifetimes have been sought.
Since the publication by Lafferty, Journal of Applied Physics, 22, pp. 299-309, March 1951, of an article describing the thermionic emission properties of alkaline and rare-earth borides having the chemical formula MB.sub.6, where M represents an alkaline or rare-earth element, and of cubic crystal structure, much effort has been expended investigating the use of LaB.sub.6 as a electron source. This material appears potentially more useful than tungsten or thoriated tungsten because its high melting temperature, low vapor pressure and small work function afford possibilities of lower operating temperatures and longer lifetimes. The lower temperature would alleviate problems of dimensional instability caused by thermal expansion or drift. Measured values for the work function of LaB.sub.6 cluster around 2.7 volts. This compares to 4.5 volts usually reported for tungsten. There is a wide range of emission values reported for tungsten and LaB.sub.6, and this variation is usually attributed to surface impurities, non-stoichiometry and the effect of averaging over several crystal planes in polycrystalline cathodes.
Several reports have been published, Applied Physics Letters 27, pp. 113-114, Aug. 1, 1975, and Applied Physics Letters 28, pp. 578-580, May 15, 1976, reporting use of LaB.sub.6 single crystals as cathode materials. Due to the ease of growing and mounting the LaB.sub.6 single crystals along the &lt;100&gt; direction, only that direction and directions approximating that growth direction have been previously investigated.
Other problems, not completely solved by the prior art, that must be overcome before LaB.sub.6 cathodes can be extensively used include chemical compatibility of the cathode with the structural mounting apparatus and dimensional stability of the crystal and mounting apparatus.